1. Field of the Invention
The present invention relates to a magnetic disk drive device for rotatingly driving a magnetic disk and a spindle motor for use in the magnetic disk drive device.
2. Description of the Related Arts
In general, a magnetic disk drive device has a spindle motor for rotatingly driving a magnetic disk in a predetermined direction and a clamp member for securing the magnetic disk to the spindle motor. The spindle motor has a hub which is rotatably supported through a bearing, a rotor magnet mounted on the hub, and a stator disposed so as to oppose the rotor magnet. The magnetic disk is secured to the hub by the clamp member.
The spindle motor used in such a magnetic disk drive device is provided with a suitable sealing means such as a magnetic fluid seal for preventing grease form coming into the space receiving the magnetic disk. The magnetic fluid seal has an annular permanent magnet and pole pieces which are mounted on both end surfaces of the permanent magnet. A magnetic fluid is charged in the ends of the pole pieces.
In known magnetic disk drive device which employs a magnetic fluid seal, however, encounters with a problem in that magnetic fluxes from the permanent magnet leaks into the space receiving the disk, causing errors in reading and writing information. This problem caused by leakage of magnetic fluxes from the magnetic fluid seal is becoming more serious when considered under the current trends for reduction in the size of the magnetic disk drive device and greater storage capacity of the magnetic disks.
Another problem encountered with the known magnetic disk drive device resides in that a loss of driving torque is increased when a lubricant of a comparatively large viscosity is used in a pair of ball bearings supporting the rotational part of the spindle motor, requiring a greater driving electric current to be supplied to the spindle motor. This problem would be obviated by the use of a lubricant having a comparatively small viscosity. Such a lubricant having a small viscosity, however, exhibits a greater tendency of scattering and, hence, a greater tendency of invasion of the space where the disk is disposed.
Furthermore, the known magnetic disk drive device requires a complicated assembly process due to difficulty encountered in correctly locating the pair of bearings at predetermined positions and at a predetermined spacing from each other.